This invention relates to intrusion detection systems and more particularly to a capacitance detection system for protection of conductive objects.
Prior capacitance detection systems generally employ some type of radio frequency oscillator having the protected object as part of the capacitance of the oscillator tank circuit and utilize changes in that circuit caused by the capacitance of an intruder to produce either a frequency shift of the oscillator or a non-oscillating condition. Various techniques are employed to detect these two conditions and to cause an alarm. A principal disadvantage of such systems is that they radiate at their operating frequency. Protected objects connected to the system function as antenna elements and thus compound the problem. Such radiation may produce interference throughout the low frequency, medium frequency and high frequency radio bands. In addition, this radiation is easy to detect by the skilled intruder.
Another disadvantage of the above system is that the design of the oscillator requires a compromise between good detection sensitivity and low false alarm rate. A low Q circuit is required for good detection sensitivity in order to produce a large frequency shift for a small capacity change whereas a stable oscillator for prevention of false alarms requires a high Q circuit. Still another disadvantage is that the sensitivity of the oscillator system is a function of the capacity load. Since the capacity change of an intruder is generally constant, as more capacity is tied to the system, its sensitivity becomes less.
This invention is directed to a capacity detection system which overcomes these problems.